Introduction

Many of us have faced a situation where we want the user to enter a number (an integer or a double) in a TextBox, thus seeking a way to prevent him from inserting invalid characters. Almost all of the available solutions required to use a custom control inherited from TextBox control. Regarding the fact that the developers sometimes (or perhaps often!) decide to add this facility to their TextBoxes after they have added several TextBox controls, and added many other lines of code to their project based on these controls names, these solutions are practically of limited use. (The programmer needs to remove all TextBoxes, add the new custom control instead of each of them, and make all the required changes to the code.)

Considering that, I broke the project to two blocks:

To validate each character a user presses on the keyboard, check if its addition to the current value of the TextBox will still result in a numeric value, and prevent the character to be added to the TextBox value if the answer is no.

To perform the validation on pasting into the TextBox. Pasting could be done through pressing CTRL+V, SHIFT+INSERT or using the right-click context menu.

The first issue can be handled using the onKeyPress event of the TextBox. For the second part, unfortunately, we don't have an onPaste event. I used a brilliant method dedicated in this Experts-Exchange thread.

Using the code

You will need to set the numericTextboxKeyPress sub to handle the onKeyPress event for all TextBoxes you want to validate. This will handle the input characters as the user types them.

The creative TextBoxOnChange class handles the paste event for the TextBoxes. For each TextBox on which you want to handle the paste event, you should add a line of code like this:

I added two checkboxes to the demo project source code to facilitate your learning how to use the code for multiple checkboxes.

History

25 Feb 2006 : Minor bug fixes.

19 Feb 2006 : Original article submitted.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

Private Sub XTextBox_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles MyBase.KeyPress ' Check for the flag being set in the KeyDown event. If acceptableKey = False Then ' Stop the character from being entered into the control since it is non-numerical. e.Handled = True Return Else If e.KeyChar = Convert.ToChar(Keys.Back) Then If strCurrency.Length > 0 Then strCurrency = strCurrency.Substring(0, strCurrency.Length - 1) End If Else strCurrency = strCurrency & e.KeyChar End If

The example from the downloaded project does not allow you to enter a decimal the format like .xx but instead 0.xx. Small point I know but many users will input a fraction this way when entering large volumes od data.

Did you know you can do the following.....
1. Press the 2 number key.
2. Open notepad and press the - key.
3. Copy the "-" text into the clipboard.
4. Past the "-" text just after the 2 in the number edit box...

As I said before, my solution is for use when programmers decide to add this facility to their TextBoxes after they have added several TextBox controls, and added many other lines of code to their project based on these controls names.
However, if you are at the beginning of the design of your forms, it is more appropriate to use a user control which is a textbox accepting numeric values, rather than using the normal textbox control and adding the feature to it as I do in this article.
You can find such controls by searching in the internet, and even in the same codeproject web site:
http://www.codeproject.com/cs/miscctrl/HDNumericTextBox.asp
http://www.codeproject.com/vb/net/2gs_txtbx.asp

>> That kind of number, although made up of numeric symbols is not really a number.
I'm not sure what you mean by the above statement. Afterall, TextBox.Text is always a string, and what I offer here, is to limit this string to contain numeric values only.

If the aim of your text box is to allow only numeric values only then you have to consider exactly what IS a number.

For example:

$4.00 is a number
50% is a number
2 is a number
5:00pm could also be considered a number

All these above examples are 'numbers' because they represent a numeric AMOUNT type of a construct.

The example that the other person gave of '123-456-789' really does not represent a numeric amount, it represents a pattern of characters that just in this case happen to all be characters that are allowed in numeric display.

Maybe I am over thinking this a bit, and if I am I am sorry to muddy the discussion here, but I think if your controls focus is to only allow numeric values then you have to consider what really IS a numeric value. Not just the individual characters but what they represent as a whole when entered in. This has a larger meaning than just creating a control that only allows numeric type characters in.

If you are only allowing numeric type characters then you have to take into account ALL the different numeric characters and I am not that sure of the overall usefulness of the control at that point.

As one further example... IPV4 addresses allow for 4 groups of numbers, each separated by a '.'. Each characters is considered a numeric type character but as whole and IP address is not considered a numeric value. Should your control allow and validate this as a proper value? If it does then it has moved from being a numeric value text box into the realm of an IP address text box.

Just trying to push home the fact that once you widen the scope of your control very wide then you open yourself up to tons of problems. If a person wants to enter in an IP address then they should use a control specifically for that type of entry. If a person wants to enter a numeric value (dollars, pounds, percents, etc...) then they use yours here. If they want to end in something else then maybe they use another. Just trying to say that you need to keep your scope on point here and be ready and willing to tell people requesting features and pointing out what 'they' think are bugs, that their request doe snot fit the scope of your control and for what reasons you feel that way. This is one reason why there are so many controls out there that end up no good. They try to be all things to all people and in the end never do anything very well.

Well in a mathematical point of view (as far as I know), $4.00 is not a number, but a number and a symbol to its left! Or an IP address is not "a" number, but a set of "four" numbers separated by a decimal sign.
However, in order to make sure such confusion is not going to happen again, I'm modifying the text of article to clearly define what the textboxes are going to do.
I hope my article to be a good start point for others to create their own controls meeting their specific needs.
Thanks for your attention

That is a point of view too. Thanks for your complements. I think those kinds of textboxes are made by other people before as seperate user controls. There are numeric textbox user controls doing what I offered here too. What I meant to offer here, was a way to prevent using a user control.